The present invention relates to a liquid crystal display element. More particularly, it relates to a polymer-dispersed liquid crystal display element having a novel composition.
The liquid crystal display elements currently used widely for display of information have many advantageous features such as low driving voltage, quick response and small thickness, but these elements, because of utilizing the polarizer film, had the drawback that they were poor in visual recognizability of their displays at a dark place.
In order to overcome this problem, the liquid crystal display elements of the so-called backlight system--a system having a light source on the back side of the display element--have prevailed recently. This backlight system, however, involves the problem of increased power consumption and is also still unsatisfactory in use under direct sunlight.
In search of a system which is capable of solving the above problem, enthusiastic studies have been made on the polymer-dispersed liquid crystal, as for instance proposed in JP-A 60-252687. The polymer-dispersed liquid crystal display devices, as compared with the devices utilizing polarized light which are most popularly used at present, have advantages in brightness, contrast, viewing angle, etc., and are also simple in structure since no polarizer is required, so that this type of liquid crystal display device is expected to not only supersede the currently used liquid crystal display devices but also find its way into a wider range of use.
Polymer-dispersed liquid crystal has a structure in which the liquid crystal phase is dispersed heterogeneously in the matrix polymer. The polymer-dispersed liquid crystal display devices which are under research at present can be roughly divided into the following two structural types: nematic curvilinear aligned phase (NCAP) structure in which the fine particles of liquid crystal are dispersed in the polymer matrix, and polymer network-liquid crystal display (PN-LCD) structure in which the particles of liquid crystal are dispersed in a spongy polymer. In the polymer-dispersed liquid crystal display devices, display is made by a change in strength of light scattering with the polymer due to liquid crystal alignment induced by refractive anisotropy of the liquid crystal molecules.
The nematic liquid crystal used in display devices differs in dielectric constant between the major-axial direction of the molecule and the direction orthogonal thereto, so that it shows birefrigence with different refractive indices for linearly polarized light against the wave fronts in the respective directions. The refractive index for polarized light in the major-axial direction is called extraordinary light refractive index (n.sub.e) and the other refractive index is called ordinary light refractive index (n.sub.O). In liquid crystal for display, usually n.sub.e &gt;n.sub.O and n.sub.e -n.sub.O =0.1 to 0.3. For the twisted nematic (TN) liquid crystal currently used for display, there are used the substrates whose surface has been pretreated for giving a twist to the liquid crystal layer, and the change of birefrigence corresponding to the change of liquid crystal alignment is effected by a pair of polarizers holding a cell therebetween. In the polymer-dispersed liquid crystal, on the other hand, the refractive index of the polymer matrix n.sub.S is equalized with ordinary light refractive index n.sub.O of the liquid crystal, and display is made by making use of the change of refractive index induced by liquid crystal alingment.
The polymer-dispersed liquid crystal has the advantages over the conventional TN liquid crystal in that it gives stronger external light as the display element changes from a light scattered state to a transparent state according to the strength of the electric field; that the viewing angle is wider because of utilization of scattering of light; and that the utilization efficiency is higher and brighter display can be obtained since no polarizer is required. It is to be further noted that use of the conventional TN liquid crystal has been limited to the devices with a relatively small display area, such as timepieces, electric calculators, etc., while the polymer-dispersed liquid crystal can be applied to the devices with a large display area, such as large-area screens, optical pulp, billboards, etc.
Since the display principle of this type of display element relies on a transmission/scattering mode based on the difference in refractive index between the polymer and the liquid crystal, this display element has the characteristics that the change of display with voltage is gentle and has no definite threshold value. Therefore, the matrix type display elements tend to cause cross-talk effects (lighting of the non-selected pixels by the leakage electric field), and have the problem that the generally used simple matrix drive is substantially infeasible, which has greatly restricted the scope of application of this type of display element.
This can be accounted for by the facts that voltage application to the liquid crystal is not necessarily performed uniformly because the ratio of liquid crystal to polymer is as high as 4.about.5 by weight, and that by-pass voltage tends to be applied to the peripheral pixels since a path through the liquid crystal is formed between the respective pixels.
The above phenomenon can be mitigated by increasing the polymer ratio. Usually, however, since the dielectric constant of the polymer is lower than that of the liquid crystal, the drive voltage elevates and the electric field tends to be concentrated at the polymer section, making it less likely to cause a sharp change of alignment of the liquid crystal molecules, with a resultant reduction of responsiveness.
The polymer-dispersed liquid crystal elements, although capable of bright display, also have the problem that the light scattering strength is insufficient when no electric field is applied. Light scattering strength can be increased by enlarging the film thickness, but this measure has the adverse effect of raising the drive voltage as well. As a technique for improving these properties, JP-A 7-261159 discloses a polymer-dispersed liquid crystal display featuring disposition of the transparent opposite electrodes (intermediate electrodes) between the polymer/liquid crystal composite layers. Also, JP-A 4-304421 discloses a liquid crystal display element which is capable of displaying neutral tints by combining a pair of polymer-dispersed liquid crystal films and driving them with the respective patterned transparent electrodes.
In the case of the conventional polymer-dispersed liquid crystal, however, since the ratio of liquid crystal to polymer is as high as 4.about.5 by weight as mentioned above, handling of the polymer/liquid crystal mixed solution is difficult especially when the said composite structure is used, and further since the polymer/liquid crystal composite films assume a liquid state, adhesion of the polymer/liquid crystal composite films to the substrates is unsatisfactory and a great deal of time and labor is required for the controlling operations in the production process, such as control of film thickness and control of disposition of the electrodes, which leads to an increase of manufacturing cost. Moreover, in the case when an ultraviolet-curing resin is used for the polymer, since generally polymer adhesion to the substrates is not good, it is necessary to closely seal the entirety of the liquid crystal cell.
Accordingly, an object of the present invention is to provide a polymer-dispersed liquid crystal display element which is capable of simple matrix drive without causing cross-talk effects.
Another object of the present invention is to provide a polymer-dispersed liquid crystal display element which is improved in contrast with no need of elevating driving voltage and also easy to manufacture.